What is Charles's Law?

Charles's Law states that for a fixed amount of gas at constant pressure, the volume is directly proportional to the absolute (kelvin) temperature. If you double the absolute temperature of a gas at constant pressure, its volume doubles. The formula is V1 divided by T1 equals V2 divided by T2, where temperatures must be in kelvin (add 273.15 to a Celsius temperature to convert).

Why must temperature be in kelvin for Charles's Law?

Charles's Law requires absolute temperature because it describes a direct proportionality between volume and temperature. At absolute zero (0 K, minus 273.15 degrees Celsius) the volume of an ideal gas theoretically reaches zero. If you used Celsius, the proportionality would not hold because Celsius zero is an arbitrary point, not the true zero of thermal energy. Always convert Celsius to kelvin by adding 273.15.

What is an example of Charles's Law in everyday life?

A hot air balloon is one of the most visible examples. Heating the air inside the balloon increases its volume (and therefore reduces its density relative to the cooler outside air), providing lift. A bicycle tyre feels harder on a hot day because the air inside has expanded. Bread dough rises more quickly in a warm kitchen because the CO2 bubbles produced by yeast expand as temperature increases.

Charles's Law Calculator

Charles's Law describes the relationship between the volume and temperature of a gas when pressure is held constant. It tells us that volume and absolute temperature are directly proportional: double the temperature and the volume doubles; halve the temperature and the volume halves. The law was established by Jacques Charles in the late 18th century and is expressed as V1 divided by T1 equals V2 divided by T2, where V1 and T1 are the initial volume and temperature, and V2 and T2 are the final volume and temperature. Crucially, temperatures must be in kelvin for this law to work. Zero on the Celsius scale is not the true zero of heat energy; the kelvin scale starts at absolute zero, which is approximately minus 273.15 degrees Celsius. To convert, simply add 273.15 to any Celsius temperature. Everyday examples of Charles's Law include hot air balloons (hotter air expands and becomes less dense, providing lift), bread dough rising faster in a warm kitchen (carbon dioxide gas from yeast expands), and a basketball becoming slightly smaller on a cold day. This calculator lets you solve for any one of the four variables. Choose which one you want to find, enter the remaining three, and the calculator returns the result. Volume can be in any consistent unit (litres or cubic metres), as long as both volumes use the same unit.

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Solve for

Initial volume V1

Initial temperature T1

Final temperature T2

Final volume V2

2.00 L

solved variable

V1/T10.003663

V2/T20.003663

Volume change+1.00 L

Temp ratio T2/T12.00

Temperature must be in kelvin (K = °C + 273.15). Charles's Law assumes constant pressure and ideal gas behaviour.

How it works

Charles's Law states V1/T1 = V2/T2 at constant pressure. Rearranging: V2 = V1 × T2/T1, T2 = T1 × V2/V1, V1 = V2 × T1/T2, T1 = T2 × V1/V2. Select which variable to solve for, enter the other three, and the calculator applies the appropriate form. The ratio V/T is constant (shown as V1/T1 and V2/T2 for verification). Both must use the same units for volume, and temperature must be in kelvin.

Worked example

A gas occupies V1 = 1 L at T1 = 273 K (approximately 0 °C). The temperature is raised to T2 = 546 K (approximately 273 °C), doubling the absolute temperature. Using Charles's Law: V2 = V1 × T2 / T1 = 1 × 546 / 273 = 2.00 L. Volume doubles as expected. V1/T1 = 1/273 = 0.003663 and V2/T2 = 2/546 = 0.003663, confirming the law. These match the default values pre-filled above.

Related calculators

Boyle's Law Calculator: solve for pressure or volume at constant temperature.
Combined Gas Law Calculator: solve for P, V, or T when all three change simultaneously.
Ideal Gas Law Calculator: use PV = nRT to find any gas state variable.
Thermal Expansion Calculator: calculate dimensional changes in solids and liquids.

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